Secondary batteries such as a lithium ion secondary battery are widely used as power sources of portable terminals, such as laptop computers, cellular phones and PDAs, which have remarkably spread in recent years. Recently, desires for extension of the operation time of portable terminals, and for shortening of the charging time thereof and the like have been increasing. Accordingly, an improvement in the performance of batteries, in particular, getting high capacity thereof and an improvement in the charging and discharging rate (rate characteristic) thereof have been intensely desired.
A lithium ion secondary battery has a structure wherein a positive electrode and a negative electrode are arranged in such a manner that a separator is interposed therebetween and the resultant is put, together with an electrolyte, into a container. The electrodes (positive electrode and negative electrode) are those that an active material for an electrode (hereinafter referred to merely as an “active material” according to the circumstances) and an optional electroconductivity supplying agent or the like are bonded to a current collector made of aluminum, copper or the like through a binder for electrode of lithium ion secondary battery (hereinafter referred to merely as a “binder” according to the circumstances). Usually, the electrodes are formed by dissolving or dispersing a binder into a liquid medium such as water or N-methylpyrrolidone (NMP), mixing an active material and so on therewith, applying the resultant slurry composition for electrode of lithium ion secondary battery (hereinafter referred to merely as a “slurry” according to the circumstances) onto a current collector, and then removing the liquid medium by drying or the like to bond the resultant as an electrode layer thereto.
The battery capacity thereof is intensely affected by the amount of the active material. The rate characteristic thereof is affected by the easiness of electron transfer. In order to improve the rate characteristic, it is effective to increase the amount of the electroconductivity supplying agent, such as carbon. In order to increase the amounts of the active material and the electroconductivity supplying agent inside a restricted space of the battery, it is necessary to decrease the amount of the binder. However, if the amount of the binder is made small, there is caused a problem that the bonding force of the electrode layer is reduced.
As a binder excellent in bonding power, there is known a copolymer made from an acrylic acid or methacrylic acid ester, acrylonitrile, and a vinyl monomer having an acid component (see JP-A-8-287915). This copolymer is insoluble in water and NMP; therefore, when a slurry is produced therefrom, the copolymer is used in the state that the viscosity thereof is adjusted to a viscosity suitable for the application thereof onto a current collector, using a thickener together.
However, the concentration of solid contents in the slurry is usually as high as 70% or more, thereby resulting in a problem that mixing of the slurry becomes insufficient or the components thereof aggregate so that the active material or the electroconductivity supplying agent are unevenly dispersed in the slurry. If the uneven slurry is used to form an electrode, the following problems arise: the ion conductivity thereof deteriorates so that the battery capacity lowers; and the bonding force of the active material is reduced so that the active material falls off from the current collector.
As a method for obtaining a slurry wherein an active material and an electroconductivity supplying agent are highly dispersed, suggested a method of mixing a paste A obtained by kneading an active material and a binder, with a paste B, obtained by kneading an electroconductivity supplying agent and a thickener, so as to yield a slurry (see JP-A-2003-45432). However, according to this method, the process therefor is complicated. Moreover, producing facilities become necessary for each of the pastes A and B and the slurry, and other large restrictions about facilities are also imposed.
There is also known a method for using, as a binder, a polymer soluble in NMP, such as polyvinylidene fluoride or polyacrylonitrile. However, the electrode produced by use of the binder is insufficient in flexibility; accordingly, its electrode layer may be cracked or fallen off when the electrode is folded or wounded and then put into a battery container.